An active filter circuit employs a combination of passive linear elements, such as resistors, inductors and capacitors, with at least one active element, such as an operational amplifier. The selection and arrangement of these elements form the basis of a transfer function for the active filter circuit.
Active filter circuits are often realized as part of an integrated circuit ("IC"). IC fabrication process and temperature variations may result in circuit parameters, such as time constants, for example, which fluctuate from IC to IC. Thus, the transfer function of these IC-based active filter circuits may not be sufficiently predictable for certain applications, including communication networks and control systems, for example.
One approach for increasing the predictability of the transfer function of an IC-based active filter circuit has been to integrate a variable impedance (i.e., a voltage-controlled resistor) within the IC-based active filter circuit. Here, the variable impedance is realized by a field effect transistor ("FET"), such as a metal oxide semiconductor FET ("MOSFET"), operating in a triode region. For the purposes of the present invention, the triode region refers to a region of operation of a FET in which its drain-to-source voltage is less than or equal to the difference between its gate-to-source voltage and a device threshold voltage. This MOSFET-based variable impedance is commonly situated at the input of the IC-based active filter circuit. By this arrangement, the drain of the MOSFET-based variable impedance is positioned to receive an input signal to be filtered, while the source of the MOSFET-based variable impedance is connected to an input of an IC-based active filter circuit. The impedance of the MOSFET-based variable impedance is adjustable in response to a control signal voltage applied between the gate and source of the MOSFET. Consequently, the MOSFET-based variable impedance may be adjusted or tuned to provide the IC-based active filter circuit with a more desirable (i.e., sufficiently more predictable) transfer function.
Ideally, the response characteristics of a MOSFET-based variable impedance would be linear. MOSFETs, however, are non-linear elements. The linearity of the response characteristics of a MOSFET operating within the triode region increases in response to an increase in the gate-to-source voltage. The response characteristics of a MOSFET-based variable impedance have been expressed using a Taylor power expansion series. This Taylor expansion series includes a linear first order term and non-linear even and odd order terms, thereafter. For the purposes of the present invention, the non-linear terms of the Taylor expansion are referred to as harmonic distortion.
For a MOSFET operating in the triode region, the second order term is the most significant non-linear term in the Taylor expansion series. Various alternatives have been proposed for the reducing the effect of the second order harmonic distortion term on the response characteristics of a MOSFET-based variable impedance. One advantageous and often commercially implemented approach, commonly referred to as the MOSFET-C integrator, proposes canceling the second order harmonic distortion term from the response characteristics of the MOSFET-based variable impedance to thereby increase its linearity. For example, see U.S. Pat. No. 4,509,019, issued on Apr. 2, 1985 to Banu et al. This solution performs this cancellation by employing a pair of substantially identical MOSFETs having a common gate. Each MOSFET is configured to operate in the triode region and function as a variable impedance (i.e., adjustable in response to a control signal voltage). More particularly, the drain of one MOSFET of the pair is designed to receive an input signal, while the drain of the other MOSFET is designed to receive a complement of the input signal. Similarly, a source of the first MOSFET is coupled with an input to an IC-based active filter circuit, while the source of the second MOSFET is coupled with a complementary input of the IC-based active filter circuit. In the response to the reception of an input signal and its complement, one MOSFET generates a second order term, which is equal in magnitude and opposite in sign with the second order term generated by the other MOSFET. Consequently, these second order terms are summed as they are input into the IC-based active filter circuit, thereby canceling each otherout.